There is a constant aim to increase the web speeds utilized in the manufacturing and finishing processes of paper, paperboard and other corresponding web-like materials in order to improve the production rate. When the web speeds are increased, it is, however, necessary to monitor the function and state of the process in a more detailed manner than before in order to avoid an increase in web breaks that impair the production rate and in various defects in the quality of the fibre web.
One method which has been found to be very efficient in the real-time monitoring of a rapidly moving fibre web and its path is to use optical diagnostic methods. Advantages of the optical methods include, for example, the possibility to take measurements of an object in a contactless manner and to take the measurements with a rapid time response. Several examples of applying optical methods in the manufacturing and finishing processes of web-like materials are known from prior art.
The present invention relates to optical imaging diagnostics for storing a spatially resolved visual image or other spatially resolved, optically measurable data about an object to be examined. In optical imaging systems, the detectors that are currently used are typically electrical matrix or line scan cameras, such as CCD cameras (charged coupled devices).
U.S. Pat. No. 5,821,990 discloses, on the principle level, a monitoring system in which measuring positions are arranged at different locations along the process to be monitored. In these measuring positions, the measuring devices used can be, for example, video cameras, and said monitoring system is suitable for use also in connection with a paper manufacturing process.
One example of commercially available optical systems, which are particularly suitable for the real-time monitoring of a fibre web and its path, are so-called web runnability monitoring (WRM) systems. These systems may comprise even several tens of camera units arranged to record an image of the fibre web and the machine means related to its processing at different points in the process. The primary function of WRM systems is to visually observe and analyze web breaks and the web runnability phenomena of the fibre web related therewith. The analysis is made by monitoring video sequences recorded in connection with said events in different camera positions along the path.
The basic principle of the WRM system is shown in FIG. 1. The camera units 1 to N may be placed, according to the need, at different points of the path of the fibre web, from the wet end of the paper machine all the way to the reeling up of the paper web. At present, single camera units used in the system are typically CCD cameras which operate on the visible wavelength range and which produce an analog video signal 10 to be transmitted to computers used as image processing units 11, 12 for image capturing, storage, digital image processing, and analysis. The results of the image analysis can be viewed via a user interface 13 placed in a control room, and the visual image produced by the camera units 1 to N can also be viewed in its unprocessed form in real time, if necessary, via video monitors placed in the control room.
Troubleshooting typically requires the examination of video samples recorded from different steps of the manufacturing process, i.e. recorded with the different camera units 1 to N. Video sequences corresponding to the same point of the moving fibre web but recorded in different camera positions 1 to N at different times can be used to find out which step in the process is the origin of the cause for a defect. For example, if a break caused by an edge defect or a hole in the web is detected in the reel-up of the paper machine (camera unit N in FIG. 1), one, must first determine if a web defect causing the break is already visible in an earlier step in the manufacturing process, that is, for example in images stored by camera units N-1, N-2. To determine this, the user of the monitoring system must find, from the video recordings of the camera units preceding the reel-up, the corresponding web section where the web defect that caused the break can be observed for the first time.
Naturally, it will be obvious that in practice, problems in the runnability of the paper web must be solved as quickly as possible to eliminate the cause of the disturbance as quickly as possible and thereby to prevent a decrease in the production or an impairment in the quality of the product.
Because of the quick movement of the web in the longitudinal direction, i.e. the machine direction, a defect in the web, which defect is visible for example in the reel-up at a given moment of time, occurs a few seconds earlier in the preceding steps of the process, for example in the press. Therefore, synchronization of the camera units 1 to N is used in the monitoring systems, such as WRM systems, to find, in the recordings of each camera position, the points always corresponding to the same area of the web in the longitudinal direction of the web. These problems involved in the movement of the web are discussed in Finnish patent application 990428, which presents a method for synchronizing image information from camera units monitoring a process, in the machine direction.
In addition to the longitudinal direction of the web, however, it is also very important to know the position of defects occurring in the web in the cross machine direction, to localize the defects in the paper web. After all, the width of the paper web may be even in the order of 10 metres in modem paper machines. However, in WRM systems and also other corresponding imaging systems, the determination of defects in the cross machine direction is, in practice, complicated by factors to be described below.
FIG. 2 shows, in a principle view, the placement of a single camera unit N of the WRM system in a paper machine, seen in the direction of the web 21. In FIG. 2, the web 21 travels in the direction indicated by an arrow, between rolls 22 and 23. In practice, because of the structure of the paper web and the conditions of the imaging location, the camera unit N must be typically placed outside the actual path of the web 21, as shown in FIG. 2. The camera unit N is thus trained on the web or another object to be monitored, either from the operating side of the paper machine (position 100 in FIG. 2) or from the so-called maintenance side (position 200 indicated with a broken line in FIG. 2). As a result, the image of the web 21 or another object, recorded by the camera unit N, is a perspective view, which makes it more difficult to find the the exact locations of the image area and the objects shown in said image in the cross machine direction of the web 21. In other words, the image of the camera unit N becomes a perspective representation, because the image is recorded at an imaging angle A to the cross machine direction of the paper web, the imaging angle A deviating from the direction C perpendicular to the transverse direction of the paper web.
FIG. 3 also shows the situation of FIG. 2 seen from the side. FIG. 3 shows that the imaging takes place at an imaging angle B to the longitudinal direction of the web 21, the imaging angle B being also typically different from the direction D perpendicular to the longitudinal direction of the paper web.
Now, said imaging angles A, B of the camera units in successive camera units 1 to N in the travel direction of the paper web, and also the other properties of the camera units 1 to N, such as the enlargement of the optics used in them, may vary from one imaging position to another; therefore, in practice, the imaging takes place from different perspectives and with different enlargements in the different imaging positions. This makes it significantly more difficult, in practice, to determine the imaging or monitoring area of the web 21 or other object accurately in the transverse direction, wherein it is also very difficult to determine the precise location of the phenomena visible in the image, in the transverse direction. In practice, images recorded as perspective representations must be interpreted, in the transverse direction of the web, subjectively according to the user's own assessment. In other words, the user evaluates, on the basis of his/her experience, the transverse location of the phenomena occurring in the images. Furthermore, it is obvious that the perspective may, in some situations, make the interpretation of the images more difficult also in the scale in the machine direction.
From prior art, also so-called web inspection systems (WIS) are known, whose principle of operation is disclosed more closely, for example, in the publication WO 01/21516. FIG. 4 shows, in a principle view corresponding to FIG. 2, the placement of camera units of the WIS system in the cross machine direction of the web 21.
In the WIS system, several camera units 40 are fixed in a camera beam 41 above the paper web in such a way that the imaging direction of a single camera unit 41 is substantially transverse to the web 21. By arranging the fields of vision of adjacent camera units 40 to be partly overlapping, the WIS system can be used to cover the width of the web 21 in the cross machine direction without significant perspective errors in the observation area, wherein it is now possible to record the precise location of web defects detected in the images in the cross machine direction.
In practice, however, a significant problem in the implementation of the WIS system and other corresponding systems is that, for example because of the space required by the camera beam 41 in the transverse direction of the web, the apparatus required by the system can only be installed in certain locations along the path of the paper web. Furthermore, the apparatus for a single imaging position will become relatively expensive, due to the large number of camera units 40. For these reasons, among other things, apparatuses complying with the WIS system are, in practice, typically installed in one imaging position only: at the final section of the paper machine, right before the reel-up.
In this imaging position, good measuring accuracy is achieved with the WIS system in the transverse direction of the web 21, because due to the placement of the camera units 40, the imaging takes place substantially without perspective errors. Furthermore, in said final section of the paper machine, the moisture of the web 21 is already settled, wherein no problems are, in practice, caused by the drying shrinkage in the transverse direction of the web.
It is obvious that the use of imaging systems of the WIS system type is primarily limited solely to the monitoring of the fibre web itself, because due to the size and structure of the apparatus, its placement to record images of other objects along the path is very difficult. In the wet end of the paper machine, the imaging with an apparatus complying with the WIS system, in which the camera units 40 are placed relatively close to the object to be imaged, would also be disturbed by e.g. water mist or water spraying from the paper web. Furthermore, the structure of the WIS system with the camera beam extending across the whole paper web would significantly encumber the service and maintenance work of the paper machine.
For the above-described reasons, it is thus typical that in the same paper machine, an apparatus of the WIS system, placed in one imaging position at the dry end, is used for measuring the properties of the fibre web itself, and in addition, a separate WRM system is used for monitoring the fibre web and the machine means (rolls, felts) involved in its processing, to troubleshoot web breaks and phenomena related to them.